Abstract

We report experimental and finite-difference time-domain simulation studies on terahertz (THz) characteristics of band gaps by using metal grooves which are located inside the flare parallel-plate waveguide. The vertically localized standing-wave cavity mode (SWCM) between the upper waveguide surface and groove bottom, and the horizontally localized SWCM between two groove side walls (groove cavity) are observed. The E field intensity of the horizontally localized SWCM in grooves is very strongly enchanced which is three order higher than that of the input THz. The 4 band gaps except the Bragg band gap are caused by the π radian delay (out of phase) between the reflected THz field by grooves and the propagated THz field through the air gap. The measurement and simulation results agree well.

Figures (4)

Experimental setup: (a) Optical micrograph of grooves. (b) Geometry of grooves. The period (p) is 142 μm, the groove width (w) is 58 μm, the groove depth (d) is 84 μm, and the number of grooves (N) is 15.

(a) Measured reference pulse for 75 μm air gap. (b) Transmitted output pulse for 75 μm air gap. (c)-(f) Measured THz spectra with various air gaps. The black and red lines indicate reference and output spectra. (g)-(j) Comparison of measured power transmission spectra (red) and FDTD simulated power transmission (Black) for spectra (c)-(f), respectively. The Roman numerals I, II, III, and IV and letters A, B, and C are used to distinguish the source of band gaps from multiple grooves and single groove, respectively.

(a) 2-dimension FDTD simulations for 15 grooves. The air gaps used in the measurement are indicated by horizontal white lines. The vertical and inclined dotted lines indicate band gap I-2 and band gap B respectively. (b) 2-dimension FDTD simulations for single groove.

E field intensity ((a)~(f)) and Hz field ((g), (h)) distribution in grooves. (a) Band gap A at 75-μm air gap with 0.7 THz single frequency. The diagram shows the out of phase state between the detour (red arrow) and straight (blue arrow) propagated THz fields. (b) Band gap B at 51-μm air gap with 2.37 THz single frequency. The diagram shows vertically localized standing-wave. (c) Band gap C at 51-μm air gap with 2.625 THz single frequency. The diagram shows horizontally localized standing-wave. (d)~(f) are identical with (a)~(c) but for single groove. (g) Band gap II-1 at 134-μm air gap with 1.4 THz single frequency. (h) Band gap II-2 at 134-μm air gap with 2.8 THz single frequency.